The present invention relates to a system and method for the absolute measurement of noise power. More specifically, the present invention relates to a method and system for the absolute measurement of noise power utilizing a gas driven shock tube as a primary laboratory standard of high temperature and of power.
Every information-carrying signal is always competing with some noise which reduces the amount of information that can be transmitted with a given signal power. Since at carrier frequencies beyond about 1 GHz the receiver itself is the most prominent source of noise, an objective measure of the extent to which the receiver rf and i.f. stages degrade the signal to noise ratio would not only provide a means of determining the ultimate threshold sensitivity of the receiver but would also furnish an important criterion for comparing the relative performance of receiver front ends. The noise factor is the only figure of merit that meets this important requirement of objectivity. To make accurate noise factor measurements, we need, as a signal, broadband noise whose temperature is both accurately known and significantly different from the equivalent noise temperature of the receiver, referred to the input terminals. (Temperatures of the order of 10,000.degree.K are sufficiently high for accurate noise factor measurements of most microwave receivers.) However, calibration of these noise sources has been one of the major problems in obtaining accurate measurements of noise factor. Existing techniques employ "hot-body noise sources" -- such as a silicon-carbide wedge in a gold waveguide or a zinc-titanate wedge in a platinum 13-percent rhodium waveguide hermetically sealed and maintained at about 1300.degree.K -- as primary standards.
There are several disadvantages associated with these techniques. The main disadvantage is that they are inherently frequency limited, with the current upper limit being 65 GHz. This limitation is primarily due to the progressive difficulty of building a load that is well matched at relatively high temperatures as the dimensions of the waveguide become smaller and smaller. Two additional disadvantages arise from the requirement that the waveguide termination be kept hermetically sealed at about 1300.degree.K, currently the highest practical temperature at which it can be maintained. First, this temperature is about an order of magnitude lower than the typical temperatures (10,000.degree.K) of the commonly used noise sources. Generally, for ease and accuracy of the measurement, we want the temperature of the standard to be comparable with that of the device under calibration. Second, maintaining a hermetic seal at temperatures as high as 1300.degree.K is not an easy task. In spite of the elaborate precautions that are taken to maintain that seal it is frequently found upon dissection of the load that an inorganic growth has formed in the waveguide and on the load, apparently as a result of some foreign matter that has leaked in. It is extremely difficult to determine the magnitude of the error that such growth introduces into the calibration system, even just prior to the dissection of the load. Thus, although these systems have theoretical accuracies of the order of 0.01 dB, we are never sure of the actual accuracy, and there is always the nagging doubt that a significant error -- e.g., 3 dB -- may be involved in the calibration. At any given time the actual accuracy can be ascertained only by dissecting and examining the termination -- a very costly and time consuming process. A fourth disadvantage is that the standard as well as the associated measuring system is relatively delicate and direct calibrations against it are relatively difficult and cumbersome to make.